Railway based passenger vehicles have used air spring secondary suspensions for a number of years. The secondary suspension provides vibration isolation for passengers and equipment in the car body, allowing for a comfortable ride. The air spring height is controlled through the use of a leveling valve which is so arranged that, as the passenger load of the vehicle changes, pressurized air is either admitted to or released from the air spring to maintain a constant air spring height, and thereby mitigate the change in car body height relative to a datum line. This is advantageous, for example, to mitigate the change in vehicle door threshold height relative to a fixed wayside platform that would otherwise occur due to air spring deflection.
Several different designs of air springs, such as rolling diaphragm and the convoluted air spring are widely available. Air springs typically have a nominal working height, with a design position range of between one and two inches. Leveling valves of various designs are also known in the art and typically use a mechanical linkage between the leveling valve and either the car body or truck, depending upon if the leveling valve is mounted to the truck or car body respectively, to control the relative height of the air spring when between the car body and the truck. As the air spring height changes, the linkage (also known as a sensing arm) causes a lever on the leveling valve to move in such a manner as to either increase the air spring pressure to compensate for an increasing load, or decrease the air spring pressure to compensate for a decreasing load, in either case restoring the original air spring height relative to its mounting. In some applications, air springs are mounted between a truck frame and truck bolster, and then the leveling valve is mounted between those two truck elements.
Railcars have a primary suspension which isolates the rail vehicle from track irregularities, and cushions the trucks and car body from the high forces generated at the wheel-rail interface. The primary suspension elements of railcars are typically rubber spring elements such as a chevron or a rolling rubber ring, or a steel coil spring either with a pedestal, radius arm or other guiding mechanism. Linear and non-linear primary suspension elements are used depending upon the design needs of the vehicle. In either case, the primary suspension is arranged to allow sufficient movement of the wheel sets for vertical wheel load equalization over track perturbations, provide a natural frequency generally less than 8 Hz, and perform other functions. Typical primary suspension deflection, depending upon the vehicle, is approximately 0.5 to 1 inch from the unloaded vehicle condition to the fully loaded vehicle condition.
In practice, car body height relative to a datum line is compensated for secondary suspension deflection, but not for primary suspension deflection. Hence, as it is desirable to maintain the vehicle passenger floor and threshold height in close alignment to the station platform to allow safe, efficient operation, and American with Disabilities Act conformance, the primary spring deflections should be compensated for, in addition to the secondary spring deflections.
Prior art teaches that primary and secondary suspension compensation can theoretically be obtained by mounting the leveling valve above the secondary suspension and connecting the leveling valve sensing arm to a link that communicates with a member that is below the primary suspension, such as the equalizing beam disclosed in U.S. Pat. No. 5,947,031 to Polley, which extends between the axle boxes of the truck wheel sets. However, due to mechanical issues created by truck rotation relative to the car body, and the high shock and vibration levels below the primary suspension, such embodiments are difficult to implement. The current invention eliminates the need for connecting to elements below the primary suspension, and provides a simple installation to allow new vehicles to be equipped with, or existing vehicles to be retrofitted with, a device which compensates for the deflection of both the primary and secondary suspension with changing passenger load.